Compression release-type engine retarders are well-known in the art. Engine retarders are designed to temporarily convert an internal combustion engine of either the spark ignition or compression ignition type into an air compressor. The fundamental braking power is achieved by preventing fuel injection during the compression stroke of a piston, compressing the captured air mass, and releasing the compressed air at or near a top-dead-center position of a piston into an exhaust manifold. The energy expended in compression release braking systems is controlled, for the most part, by the volume of gas compressed, the timing of the release of the gas into the exhaust manifold and the amount of gas released. A compression release retarder decreases the kinetic energy of an engine by opposing the upward motion of the engine's pistons on the compression stroke. As a piston travels upward on its compression upstroke, the gases that are trapped in the cylinder are compressed. The compressed gases oppose the upward motion of the piston. When the piston nears the top of its stroke, an exhaust valve is opened to "release" the compressed gases. The pressure having been released from the cylinder, the piston cannot recapture the energy stored in the compressed gases on the subsequent expansion downstroke. In so doing, the engine develops retarding power to help slow down the vehicle. This provides the operator with increased control over the vehicle.
A properly designed and adjusted compression release-type engine retarder can develop retarding power that is a substantial portion of the power developed by the engine on positive power. Compression release-type retarders of this type supplement the braking capacity of the primary vehicle wheel braking system. In so doing, these retarders may substantially extend the life of the primary wheel braking system of the vehicle.
The basic design of a compression release type engine retarding system is disclosed in U.S. Pat. No. 3,220,392 to Cummins, which is incorporated herein by reference. The compression release-type engine retarder disclosed in the Cummins patent employs a hydraulic control system to operate the exhaust valves to effect the compression release event. The hydraulic control system engages the engine's existing valve actuation system, namely, the rocker arms of the engine.
When the engine is operating under positive power, the hydraulic control system of the compression release retarder is disengaged from the valve actuation system, so that no compression release event occurs. When compression release retarding is desired, the engine is deprived of fuel and the hydraulic control system of the compression release brake engages the valve actuation system of the engine. The valve actuation system drives the compression release retarder to produce compression release events at the appropriate times.
The hydraulic systems of compression release engine retarders typically have a number of components. A master piston engages the valve control system of the engine, typically at a rocker arm. A solenoid valve is typically actuated to supply engine oil to fill the hydraulic circuits of the compression release engine retarder, when retarding is desired. The master piston, in turn, is hydraulically connected to a slave piston. The slave piston is connected to an exhaust valve of the engine.
When the compression release retarder is actuated, the rocker arm pushes against the master piston. The motion of the master piston forces the slave piston to actuate, which in turn opens the exhaust valve of the internal combustion engine at a point near the end of the compression stroke. Much of the energy stored by compressing the gas in the cylinder is not recovered during the subsequent expansion or power stroke of the engine. Instead, it is dissipated through the exhaust and radiator systems of the engine. By dissipating the energy developed by compressing the cylinder charge, the compression release-type retarder slows the vehicle down.
Typically, it is desirable to use the compression release-type engine retarder to open an engine exhaust valve as late in the compression stroke as practical. In this way, the engine develops greater compression, allowing more energy to be dissipated through the compression release retarder. Delaying the opening of the exhaust valve in the compression release event, however, may substantially increase the loading on critical engine components. The force required to open the exhaust valve during the compression release event is transmitted back through the hydraulic system through the push tubes and the camshaft. This can impose substantial force on certain engine components. If the timing is delayed long enough, the pressure in the cylinder can become high enough to exceed the ability of the compression release retarder to properly open the exhaust valve.
In a compression release engine retarder it is desirable to provide accurate timing of exhaust valve opening. To this end, it is advantageous in these systems to apply sharp hydraulic pulses to the slave pistons so that they open the exhaust valves rapidly. In order to both stop the slave pistons' motion and prevent excessive opening of the associated exhaust valves, reset or "clipping" mechanisms have been employed to reduce the hydraulic fluid pressure when either the hydraulic fluid pressure reaches the predetermined maximum or the slave pistons have reached the end of their desired stroke. The term "clipping" generally refers to modification of the forward motion of the slave piston to control peak brake pressure, to limit the total travel of the slave piston or to reduce the length of the slave motion event. The disadvantages of excessive slave piston travel include excessive exhaust valve travel and possible contact of exhaust valves with the engine piston, increased overall braking apparatus and engine height, and overtravel of the slave piston return spring.
A typical slave piston design incorporating such a reset mechanism uses a lash-adjusting screw containing a reciprocating plunger that makes a facet over a hole in the slave piston surface. A means for adjusting the lash--or cold-engine clearance between the slave piston and the valve stem--is desirable in most valve actuation systems. In the typical design, the travel of the reciprocating plunger is arrested upon contact with a press-in pin or retaining ring that fits in a slot within the body of the screw. However, this system is relatively costly to manufacture and assemble due to the complex configurations of its various parts, the need to test it to ensure that the pin or retaining ring will not come out, etc. Failure of the retaining ring is a problem in such prior art devices.
The lash adjustment screw in prior art devices is typically contained in a housing at the top of the slave piston cylinder. Because the lash adjustment screw position is variable, the slave piston cylinder manufacturing datum, or reference position, is not fixed. This makes the device difficult to manufacture to a tightly controlled clipping position specification. The lash-adjusting screw may also cause a problem if it is hollow at the point at which it intersects the housing or other mounting because it may break if tightened excessively.
Other devices have been tried to "clip" the forward stroke of the slave piston to, for example, prevent excessive travel of the associated engine exhaust valves which cause them to contact the top of the engine piston at or near its top dead center position. Mechanisms for producing such slave piston clipping are shown, for example, in U.S. Pat. No. 5,511,460 to Custer, U.S. Pat. No. 5,201,290 to Hu, U.S. Pat. No. 5,787,859 to Meistrick et al. and U.S. Pat. No. 5,809,964 to Meistrick et al, all of which are assigned to the same assignee as the present invention and are incorporated herein by reference. These mechanisms typically include components for opening an aperture in the slave piston when the forward stroke of the slave piston has progressed to a predetermined point. This allows hydraulic fluid to bleed from the slave piston cylinder, thereby preventing the slave piston from moving forward beyond the point at which the aperture is open. The problem with prior slave piston clipping mechanisms is that they typically employ adjusting screws having tight tolerances which are difficult and expensive to manufacture. U.S. Pat. Nos. 5,787,859 and 5,809,964 to Meistrick et al. disclose stroke limiting in connection with a bifurcated slave piston, in contrast to the simpler, single slave piston of the present invention.
The slave piston clipping device of the present invention may be used in conjunction with other devices which alter, advance or retard the movement of a slave piston, such as the device disclosed in U.S. Pat. No. 5,186,141 to Custer, which is assigned to the same assignee as the present invention and is incorporated herein by reference.
Thus, investigation is still ongoing to find a simple, cost-effective method to limit the travel of a slave piston in a compression release engine retarder.